Why was support for Tuple spaces removed completely?

[michalgregor]

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Hi, everyone, it seems that this commit introduced the following check into BaseVectorEnv:

if isinstance(obs_list[0], tuple):
            raise TypeError(
                "Tuple observation space is not supported. ",
                "Please change it to array or dict space",
            )

I've seen that there was some discussion previously about Tuple spaces not really being a good fit for the way that Batch works. I am not sure, whether anything changed in this particular PR that precludes Tuple spaces from working altogether, but previously I was able to make them work (I've built a class that processes the batch into the correct format for the PyTorch module 🤷‍♂️).

If it is just that using them is considered messy / to be discouraged, maybe it's a bit extreme to raise an exception? Couldn't we turn it into a warning or something? Or is there a deeper reason behind the change?

[Trinkle23897]

It's because if we use tuple action space together with Batch, there are tons of unpredict behaviors and can cause potential bugs. If we only give use a warning instead of explicitly raising error, it's hard for user to find out such kind of a bug.

[michalgregor]

I see – does this really resolve the issue then though? You can still have Tuple spaces nested under a Dict space, no? That will not have the same problems?

[Trinkle23897]

No, we recommend to use nested Batch with numpy array instead of batch+tuple, since numpy array has type system.